One technique which is commonly used for charging Valve Regulated Lead Acid (VRLA) batteries, which are typically used in standby applications such as emergency systems, is known as float charging. However, it has been found that there are a number of drawbacks associated with float charging. These include the fact that the float charging regime overcharges the battery, and also causes excessive temperatures of the battery cells, which in turn degrades the chemical composition of the electrolyte in the battery, so as to shorten the battery life.
A more efficient battery charging principle for standby VRLA batteries involves the use of what is known as the Intermittent Charge Control (ICC) principle, as described in reference M Bhatt, W G Hurley, W H Wölfle, “A New Approach to Intermittent Charging of Valve-Regulated Lead-Acid Batteries in Standby Applications”, IEEE Transactions on Industrial Electronics, vol. 52, no. 5, pp. 1337-1342, October 2005.
The ICC principle consists of four distinct operating modes, as shown in FIG. 1. In Mode 1, a battery is charged with a charge current of 0.1Crated A, where Crated is the rated battery capacity in Ah. The purpose of Mode 1 is to charge the battery to a high state of charge (SOC) of over 85%. When the battery voltage triggers the upper threshold voltage (Vut), the operating mode changes from Mode 1 to Mode 2. In Mode 2, the battery is kept at open circuit to reduce the battery internal resistances built up in Mode 1, and provide more voltage head room for Mode 3. When the battery voltage drops below a lower threshold voltage (Vlt), the operating mode changes from Mode 2 to Mode 3. In Mode 3, the battery is charged with pulsed-currents with a peak current value of 0.05Crated A, a period of 30 seconds and a current duty cycle (D) of 33.3%. The purpose of Mode 3 is to charge the battery to full SOC. When the battery voltage reaches the upper threshold voltage again, the operating mode proceeds to Mode 4. In Mode 4, the battery is fully charged and it is kept at open circuit. The battery voltage drops due to self-discharging in Mode 4. When the battery voltage drops below a restart charge voltage threshold (Vr), which indicates an SOC of 97%, the charging cycle is restarted starting with Mode 1.
The advantage of the ICC principle is that this charging regime prevents the battery from overcharging, while at the same time keeping the battery at high SOC to prevent sulfation.
However, it has been found that the reaction rate in the electrolyte doubles for every 10° C. increase in temperature. This in turn causes corrosion at the positive grid, and increases water loss and generates extra heat, which could lead to thermal runaway.
Temperature compensation schemes exist for the float battery charging method. This involves the adjustment of the float voltage to prevent thermal runaway when the temperature is high, and prevent cell self-discharge when the temperature is low. The temperature compensated battery charger adjusts the float charging voltage based on the sensed ambient temperature or battery temperature. When the temperature increases, the charging voltage is accordingly decreased.
However, no temperature compensation technique is currently provided under the ICC principle. It is therefore an object of the present invention to provide a temperature compensation for pulsed battery-charging schemes, such as those using the ICC principle, in order to improve the battery lifetime.